THE HARMFUL AND INCREASING EFFECTS PRESENTED BY SCTLD
Introduction
Sea turtle- just one animal who depends on coral reefs for survival
Coral reefs are arguably the most influential marine life contributing to a healthy ocean because they provide shelter, nursing, reproduction, and feeding ground for countless numbers and varieties of marine species. Coral reefs are the core and foundation of creating a healthy balance between all marine life in the ocean, for this reason if coral reefs are suffering, it will result in an unbalanced ecosystem.
Stony Coral Tissue Loss Disease (SCTLD) is on its way to becoming one of the most deadly coral diseases in history, affecting countless Caribbean reefs. SCTLD was first identified in Florida in 2014 and was seen again in 2018 where it had spread to Mesoamerica, affecting nearly 300 miles of reef in only a few months. This disease affects over twenty species of coral in the Caribbean. However, with the destruction of healthy coral reef systems, many more species of marine animals will suffer since coral reefs provide support to roughly 25% of marine life (Basic Information About Coral Reefs, United States Environmental Protection Agency). SCTLD causes bleach or discoloration in a coral’s tissue through two types of cellular injuries:
1. Lytic lesions, this is when the tissue is completely dissolved and becomes a gooey, gummy-like mass.
2. Coagulative Necrosis, this is when the tissue maintains its shape and structure but loses its nuclear matter.
This disease also appears to be more serious during cooler months (Stony Coral Tissue Loss Disease [SCTLD], Coral Disease Health Consortium).
Although the specific cause of SCTLD is still unknown and being studied, we do know this disease can be transmitted through direct contact and via a waterborne agent. Therefore, contact with water from an infected reef site will transmit the disease to an uninfected reef site. Unfortunately, cruise ships play a big role in spreading this highly infectious and deadly disease. A cruise ship will take on water (ballast water) to maintain its stability and then will dump the water when it is no longer necessary. Due to the transportation of water, it is highly possible the water contains the agent causing SCTLD (Study Links Spread of Deadly Coral Disease to Ship Ballast Water, Atlantic Oceanographic & Meteorological Laboratory). It is also unknown what the long term post-disease effects will be on surviving coral, as SCTLD weakens and threatens a coral’s ability to maintain a healthy ecosystem and a structured reef. A study was conducted along a 280 mile reef stretch in the Mexican Caribbean to test the severity of SCTLD and the affected corals’ ability to be functional and contribute to the coral community.
Methods and Results
Healthy reef
Between July 2018 to January 2020, 29,095 colonies were observed– 17% were recently dead, and 10% were already infected by SCTLD. Twenty-five of forty-eight surveyed species were affected. Dendrogyra cylindrus, otherwise known as pillar coral, and Meandrina spp., otherwise known as cauliflower coral, species from the Meandrinadae family, experienced disease pervasiveness and mortality greater than 80%. Data also revealed that less noticeable species were not nearly as functional post-outbreak compared to pre-outbreak. However, post-outbreak data suggests that these species were not affected as seriously. This unmatched data is most likely caused by the fact that these smaller and less visible species were down to their skeletons and abundantly covered in algae, which caused them to be inaccessible during post-outbreak observations. There was about a 30% reduction rate in coral communities’ ability to produce calcium carbonate (the material that allows for a reef to have structure). Data showed that depth, reef zone, structural complexity, or coral density do not have an effect on the severity or spreading of the disease. However, areas closer to more advanced and developed coasts were affected more seriously than isolated reefs. Wind exposure also caused an increase in disease (Stony Coral Tissue Loss Disease Decimated Caribbean Coral Populations and Reshaped Reef Functionality, Communications Biology)
Discussion
In this study, we see that different species of coral were affected differently by this disease. Species belonging to the Meandrinadae family and subfamily Faviinae were generally affected the worst by SCTLD. The functionality of these corals post-outbreak also significantly decreased. The corals being affected by this disease provide structure to a reef and without a strong base, reefs will be unable to build a stable environment. Since depth, reef zone, structural complexity, or coral density do not have an effect on the severity or spreading of the disease, we can conclude that the presence and capacity of the SCTLD agent is the cause for infection and increase of severity. Areas that were closer to more developed coasts were more seriously affected possibly for a few reasons:
1. The agent comes from a material produced on land.
2. Reefs are already under more stress and strain closer to busy coasts, making them more vulnerable.
3. Ships transport the SCTLD agent from a further site through ballast water that is released on reefs close to shore.
Wind exposure also caused an increase in disease likely because the wind causes an increase in currents, which result in the spread of disease faster and further.
An issue brought up during this study was that data did not line up when observing smaller, less noticeable coral. When being surveyed, these corals pre-outbreak compared to post-outbreak did not seem strongly affected, however, there was a noticeable decrease in their contribution to the reef which suggested that they were affected more than the data said. This misalignment points to the fact that there were data pieces missing somewhere along the way. Scientists concluded that some of these smaller corals had died and became covered in algae, preventing researchers from having access to them.
In the future, research should be geared towards what specifically is causing this disease such as understanding whether the disease is caused by a land produced substance or bacteria. Conducting this experiment in a controlled and easily accessible environment, such as a contained, controlled simulated aquatic tank, could help pinpoint the cause of this disease. Since we know that the severity and cause of infection is solely in the capacity of SCTLD agents, coral living in salt water is the only necessary ingredient. If SCTLD infected water is transferred to this tank, it is possible to have a constant eye on what is happening to the coral. From here, researchers can even try and separate the agent by conducting the same experiment in different circumstances, such as using fresh water instead of salt water, adding pollution, adding fish, etc., and taking note of the varying effects the agent has on the coral through these altered circumstances.
References
US EPA, OW. “Coral Reefs.” Www.epa.gov, 21 May 2023, www.epa.gov/coral-reefs#:~:text=Coral%20reefs%20are%20among%20the.
“Stony Coral Tissue Loss Disease (SCTLD).” Coral Disease & Health Consortium, cdhc.noaa.gov/coral-disease/characterized-diseases/stony-coral-tissue-loss-disease-sctld/.
Communications, AOML. “Study Links Spread of Deadly Coral Disease to Ship Ballast Water.” NOAA’s Atlantic Oceanographic and Meteorological Laboratory, 23 Nov. 2022, www.aoml.noaa.gov/ship-ballast-deadly-coral-disease/.
Alvarez-Filip, Lorenzo, et al. “Stony Coral Tissue Loss Disease Decimated Caribbean Coral Populations and Reshaped Reef Functionality.” Communications Biology, vol. 5, no. 1, 9 June 2022, https://doi.org/10.1038/s42003-022-03398-6.